Chapter 5 Chemical Reactions
I. 5.1 Nature of Chemical Reactions
II. 5.2 Reaction Types
III. 5.3 Balancing Chemical Equations
I. 5.1 Nature of Chemical
Reactions
A. Signs of a chemical reaction
1. bubbles are given off
2. changes in color
3. precipitates are formed
4. energy is released or absorbed
B. Chemical reactions rearrange
atoms
1. Reactants - a substance that
undergoes a chemical change
a. bonds are broken
b. energy is added in the form of
–
–
–
–
(1) heat
(2) electricity
(3) sound
(4) light
2. Products - a substance that is the
result of a chemical reaction
a. bonds are formed
b. energy is released in the form of
– (1) heat
– (2) light
– (3) sound
3. Reactions can be referred to as
a. Exothermic - a reaction that transfers
energy from the reactants to the
surroundings usually as heat
b. Endothermic - a reaction in which energy
is transferred to the reactants usually as
heat from the surroundings
II. 5.2 Reaction Types
A. Classifying Reactions
1. We use patterns to identify the
general types of chemical reactions
a. Synthesis reactions
– (1) a reaction of at least two substances that form a
new, more complex compound
– (2) general form
» (a) A + B g AB
» (b) 2 Na + Cl2 g 2 Na Cl
» i) subscript indicates the number of atoms per
molecule
» ii) coefficients indicates the number of
molecules
» iii) g is read as yields
b. Decomposition Reactions
– (1) a reaction in which one compound breaks into
at least two products
– (2) general form
» (a) AB g A + B
» (b) 2 H2O g 2 H2 + O2
– (3) electrolysis is the decomposition of a
compound by an electric current
c. Single Replacement (displacement)
Reactions
– (1) a reaction in which atoms of one element take
the place of atoms of another element in a
compound
– (2) general form
» (a) XA + B g BA + X
» (b) 3 CuCl2 + 2 Al g 2 AlCl3 + 3 Cu
– (3) combustion a reaction in which a compound
and oxygen burn
» (a) 2 CH4 + 4 O2 g 2 CO2 + 4 H2O
d. Double Replacement (displacement)
Reactions
– (1) a reaction in which a gas, a solid precipitate,
or a molecular compound is formed from the
apparent exchange of ions between two
compounds
– (2) general form
» (a) AX + BY g AY + BX
» (b) Pb(NO3)2 + K2CrO4 g PbCrO4 + 2 KNO3
e. Reduction/Oxidation (redox) reaction is a
reaction that occurs when electrons are
transferred from one reactant to another.
– (1) reduced - substance that accept electrons
– (2) oxidized - substance that gives up electrons
III. 5.3 Balancing Chemical
Equations
A. Four Steps for Balancing Chemical
Equations
1. Determine the type of reaction.
a. D - Decomposition
b. S - Synthesis
c. SR - Single Replacement
d. DR - Double Replacement
2. Recombine ions/atoms on the product
side.
a. different free elements and different
compounds
3. Balance compounds on the product
side
a. using subscripts
b. check for diatomic elements
– (1) only for Decomposition and Single
Replacement reactions
4. Balance equation by placing coefficients
in front of each compound or free element
where necessary
a. Conservation of matter
b. must have the same number of atoms for
each element on both sides of the equation
5. Examples
a. Mg + O2 g
S
2 Mg + O2 g 2 MgO
b. Pb(NO3)2 + KI g
DR
Pb(NO3)2 + 2 KI g PbI2 + 2 KNO3
c. N2 + H2 g
d. NaCl g